1. Field of the Invention
The invention concerns a laminate of thermoplastic film materials which exhibits throughgoing porosity, in more detail a porosity which at least over a part of the passage of a fluid from one surface to the other has the character of microporosity. Microporous film or sheet materials, including laminates, find extensive use, in particular as “house wrap film”, roof underlay and apparel which protects against rain, while allowing moisture to dissipate.
For such applications the resistance to hydrostatic and dynamic water pressure combined with easy passage of air and water vapor, as well as yield tension, puncture strength and tear propagation resistance are particularly important. For use as roof underlay it is generally considered that the resistance to hydrostatic water pressure must be at least about 60 cm preferably as much as 100-200 cm, water, while the requirement for simpler “house wrap film” is lower. Present materials used for the higher quality markets comprise fibrous sheets formed by flash-spinning (Tyvek®) and laminates of fabrics and microporous film. The manufacturing costs for these products are rather high. For simpler “house wrap film” cheaper materials are used e.g. laminates of a meltblown fibrous web and non-woven fabrics or reinforced films with perforations in the range about 50-100 μm.
It can be expected that the market for film or sheet material with throughgoing porosity but relatively high resistance to hydrostatic pressure and good strength properties can be extended to other groups of commodity articles e.g., to “breathable” industrial bags, if the manufacturing costs are reduced.
2. Description of the Related Art
The term “bosses” as used hereinafter refers to a distinctly protruding portion of the surface of a polymer film, the smallest dimension of which, measured on the film surface is at the most a few millimeters. The distance between such protruding surface portions is at the most, a few millimeters. The protrusion must be distinct but need only amount to a fraction of the film thickness.
The general concept of forming a throughgoing porosity in a film laminate under use of an interfacial channel system, which connects mutually displaced series of perforations in the plies, is known from the applicant's earlier invention, published in WO-A-04/54793, see especially FIG. 13. However, in the disclosure in this patent application, there are no constrained, interfacial channel parts as in the present application, the interfacial channels being formed by flutes alone. As a consequence the resistance to hydrostatic pressure is very low, while the resistance to dynamic water pressure (to rain storm) is high and the possibilities for ventilation through the laminate are particular good. As it appears from the said publication, some constrain can be introduced by addition of a fibrous layer between the fluted plies, which however is not as simple and not as efficient as the establishment of the above mentioned barrier parts of the interfacial channel system. While according to the invention the gap must be limited to generally about 200 micrometers or less, it is usually preferable that it be no more than generally about 100 micrometers, and when a resistance against 100 cm hydrostatic pressure is required, a gap of about 50 micro-meters or less may be needed. The gap may even be as narrow as a few micrometers, but formed distinctly as will be described later.
However, generally the average gap should be no less than 3 μm, preferably no less than 5 μm, over a majority of the area which forms the barrier part. For most purposes such as roof underlay, “house-wrap-film”, waterproof apparel, sanitary products, breathable bags and other potential uses, in which good ventilation or vapor transmission is requested, the said average gap should preferably be no less than 10 μm.
Spot-or linebonded laminates, in which the piles are perforated with the perforations in different piles mutually displaced, are also known from other patent publications, as follows.
GB-A-1075891 (Kaaber) concerns a laminate for packing purposes, consisting of two or several layers of a polymer sheet or film, provided with holes for allowing passage of gases and vapor, in which the holes in at least one of the layers are covered by non-perforated material of another layer, the lamination being only partial so as to leave a passage from each hole in one layer to at least one hole in the adjoining layer. Preferably the individual layers consist of substantially unidirectionally orientated films made from highly crystalline polymers, the direction of orientation varying in adjoining layers in the laminate. In this patent, there is not taken any precaution to secure or control the passage of gases or vapor between two adjacent piles from holes in one ply to holes in the other ply, and such passage will depend on accidentally occurring surface irregularities or small lamination irregularities.
DE-A-3245195 concerns a method of manufacturing webs of material coated with a plastic film, especially coated paper or coated non-wovens. The webs are supplied with micro-perforations, preferably by means of electrical sparks. The field of uses are in particular sanitary items. In an embodiment there is used two such coated and microperforated webs, and the microperforations in the two webs are mutually displaced. It is disclosed that there can be distance between the two webs with the purpose to set-up a further resistance to the passage of fluid, but there is no teaching of how this distance is achieved, neither is anything mentioned regarding the bonding system.
U.S. Pat. No. 4,567,080 (Korsgaard) concerns a vapor barrier comprising two vapor-tight layers. The openings in one layer are arranged in displaced positions relative to the openings in the other layer. The vapor barrier offers a resistance to the diffusion of water vapor through the vapor barrier from one side towards the other, and enables condensed water formed on the other side of the vapor barrier to travel in the opposite direction and to be evaporated from the first mentioned side. This product is mainly thought of as a vapor barrier for use in connection with roof construction comprising a vapor-tight outer covering. The holes have relatively large dimensions, e.g. (if they are circular) the diameter in the layer which is placed interiorly can be 20-30 mm, while the diameter of the other (exterior) layer can be about 5. Due to the water absorbing layer between the two layers of vapor-tight material it is clear that this product construction does not exhibit any resistance to hydrostatic water pressure, a property which the market today requests.
US-A1-2003/0165663 concerns a gas permeable polymeric film laminate comprising two perforated polymeric films bonded together such that there are gas passage between the films from perforations in one film to perforations in the other film, the perforations having a minimum dimension of at least 20 μm and the passages having a maximum dimension of not more than 15 μm. Such films laminates are proposed for use in form of packages for medical equipment, with the affect that they act as bacteria filters, while allowing sterilant gas to go through the film. The only method which is disclosed for the establishment of the low distance between the two films is by laminating the latter under use of an appropriate adhesive applied in a suitable pattern which allows the flow of gas from holes in the other film, and hereby the distance between the two films is determined by the thickness of the adhesive. However, it is difficult to adjust this thickness under industrial conditions especially when making wide webs, since it requires precision in the construction of the laminating rollers, to squeeze the applied adhesive evenly and with correct thickness over the full length of the rollers.